As one important type of flat plate display, a LCD, such as TFT LCD, has been developing rapidly in the last decade and has attracted the attention. Due to intensive competition among the manufactures and advancement in manufacturing technology of TFT LCD, LCDs with excellent display performance and lower price have been increasingly put into market. Therefore, introduction of more advanced manufacturing technology to simplify the production process and reduce the production cost has become an important guarantee for the manufacturer to survive in the intensive competition.
The manufacturing technology for TFT LCD array substrate has undergone the map from the seven mask (7Mask) technology to the current five mask (5Mask) technology, and the 5Mask technology today has become the mainstream for manufacturing TFT LCD array substrate.
Some manufacturers have attempted to exploit the four mask (4Mask) technology in fabrication. This 4Mask technology is based on the previous 5Mask technology, in which the mask for forming active layer (Active Mask) and the mask for forming source/drain electrode (S/D Mask) are merged into a single one with the aid of gray tone mask, and the functions of the original two masks, i.e., Active Mask and S/D Mask, are achieved by the single mask through modification to the etching processes.
Gray tone mask has a slit-shaped pattern thereon, and partially transparent patterned regions are formed on the mask due to the interference and diffraction of light passing the patterned regions of the mask. During exposure, the light only partially passes through the partially transparent portions. By controlling the exposure quantity, light passing through the partially transparent portions illuminates portions of photoresist and has the portions partially exposed, and the light passing through the remaining fully transparent portions of the mask illuminates the other portions of the photoresist and has these portions fully exposed. After developing, no photoresist exists in the fully exposed regions, and photoresist thickness in the partially exposed regions is less than that in the non-exposed regions, so that the exposed photoresist is shaped in three-dimension. The photoresist thickness can be controlled by controlling the transmittance ratio among the regions of the gray tone mask, i.e. the “duty ratio” of the slit region to the empty region. The method of forming a three-dimensional pattern with different thickness on the photoresist through a mask with partially transparent pattern is collectively called gray tone mask technology.
The conventional 5Mask technology uses five masks for photolithography, including the masks for forming gate electrode (Gate Mask), active layer (Active Mask), source/drain electrode (S/D Mask), via hole (Via Hole Mask), and pixel electrode (Pixel Mask), respectively. The processes using the respective masks further include one or more thin film deposition and etching process (e.g., dry etching or wet etching process), thus resulting in five cycles of thin film deposition, photolithography, and etching processes, as shown in FIG. 2.
A typical pixel unit of a TFT LCD array substrate manufactured by the above conventional 5Mask technology is shown in FIG. 1.